1. Field of the Invention
The present invention relates to the technical field of reinforcing materials, adapted to the constitution of composite pieces. More precisely, the invention relates to a novel intermediate material constituted by an array of ribbons of unidirectional fibres for making composite pieces, by subsequent injection or infusion of thermosetting, thermoplastic resin, or a mixture of the two, a manufacturing process of such intermediate material, as well as a manufacturing process of composite pieces from such intermediate material, and the resulting composite pieces.
2. Description of Related Art
The manufacture of composite pieces or articles, that is, comprising on the one hand one or more reinforcements or fibrous tapes and, on the other hand, a matrix mainly of thermosetting (<<resin>>) type and capable of including thermoplastic, can for example be made by a so-called “direct” or “LCM process” (from the English <<Liquid Composite Moulding>>). A direct process is defined by the fact that one or more fibrous reinforcements are used in the “dry” state (that is, without the final matrix), the resin or matrix being used separately, for example by injection in the mould containing the fibrous reinforcements (“RTMI” process, from the English term Resin Transfer Moulding), by infusion through the thickness of the fibrous reinforcements (“LRI” process, from the English <<Liquid Resin Infusion>> or process “RFI”, from the English <<Resin Film Infusion>>), or else by manual coating/impregnation by roller or brush on each of the unitary layers of fibrous reinforcement, applied successively to the form.
For the RTM, LRI or RFI processes, in general a fibrous preform of the form of the planned finished article should be made first, then this preform should be impregnated with resin. The resin is injected or infused by differential of pressures and temperature, then once the entire quantity of necessary resin is contained in the preform, the whole is brought to a higher temperature to complete the polymerisation/reticulation cycle and cause curing.
The composite pieces used in the automobile, aeronautics or naval industry are in particular subjected to very strict demands, especially in terms of mechanical properties. The mechanical properties of the pieces are mainly linked to a parameter which is the volume rate of fibres (TVF).
In these sectors, a large number of preforms is made based on reinforcing material, mainly of carbon fibres, especially of the unidirectional type. It is possible to theoretically calculate the maximal volume rate of fibres contained in a unidirectional tape by supposing two types of arrangements: hexagonal or square. Supposing respectively an arrangement of hexagonal type and an arrangement of square type, the maximum TVF obtained is respectively 90.7% and 78.5% (An Introduction to Composite Materials, D. Hull, T. W. Clyne, Second Edition, Cambridge Solid State Science Series, 1996). But in reality it seems difficult to get volumic fractions of fibres over 70% for composite pieces. In practice, it is commonly admitted by the expert that a volume rate of fibres (TVF) of around 60% is standard for making satisfactory composite pieces, especially with good reproducibility (S. T. Peters, <<Introduction, composite basics and road map>>, in Handbook of Composites, Chapman & Hall, 1998, p. 1-20 and in particular p. 8).
The resin which is subsequently associated, especially by injection or infusion, with unidirectional reinforcing tapes during production of the piece can be thermosetting resin, for example of epoxy type. For proper flow through a preform comprising a stack of different layers of carbon fibres, this resin is most often highly fluid, for example with a viscosity of the order of 50 to 200 mPa·s. at infusion/injection temperature. The major disadvantage of this type of resin is its fragility after polymerisation/reticulation, which causes low resistance to the impact of composite pieces produced.
To resolve this problem, it has been proposed in documents of the prior art to associate unidirectional layers of carbon fibres with a veil of thermoplastic fibres. Such solutions are especially described in patent applications or patents EP1125728, US 628016, WO 2007/015706, WO 2006/121961 and U.S. Pat. No. 6,503,856. The addition of this veil improves the mechanical properties in a compression test after impact (CAI), a test currently being used to characterise the resistance of structures to impact.
In prior patent applications WO 2010/046609 and WO 2010/061114 the applicant has also proposed particular intermediate materials comprising a tape of unidirectional fibres, carbon in particular, associated by adhesion, on each of its faces to a veil of thermoplastic fibres (also called non-woven), as well as their preparation process. The unidirectional tapes ensuring total cover, for some applications especially for making pieces of substantial thickness, one of the limitations of this type of intermediate material is its low transverse permeability to resin which will be injected or infused to constitute the final piece. In this context, a solution has been proposed in patent application WO 2010/046609 consisting of making holes extending into the material, allowing it to have a given opening factor. This solution can be used in practice on tapes of width greater than around 20 mm, and preferentially greater than 50 mm, though is more difficult to implement in the case of threads/tape of lesser widths. Widths of 3.17 mm or 6.35 mm are used for example in automatic deposit on existing machines and therefore present a particular interest.